1. Field of the Invention
The present invention relates to a process of and an apparatus for heat-treating compound semiconductors and a compound semiconductor heat-treated by the process, and more particularly, to a process of and an apparatus for heat-treating II-VI compound semiconductors (expressed by the group in the Periodic Table) and a II-VI compound semiconductor heat-treated by the process.
2. Description of the Background Art
Zinc selenide (ZnSe)-based light-emitting devices have been attracting attention as laser diodes or light-emitting diodes (LEDs) that emit short-wavelength light such as blue, green, and bluish green. These light-emitting devices can be produced on a gallium arsenide (GaAs) substrate, which is made of a material different from ZnSe. However, to increase the crystallinity of a ZnSe-based thin film formed on a substrate and to improve the device property, it is desirable that the ZnSe-based thin film be formed by homoepitaxial growth on a ZnSe substrate.
Recent years have seen the development of technology for producing a white LED by a novel concept in which white light is obtained by mixing short-wavelength light emitted by an active layer formed on a ZnSe substrate and long-wavelength light emitted by the ZnSe substrate itself excited by the light emission of the active layer. In the case of the white LED, it is essential to use a ZnSe substrate for utilizing the light emission of the substrate itself.
As a result, a single-crystalline ZnSe substrate is used as the substrate for a light-emitting device. To simplify the structure of the device for improving the device property, it is essential to increase the electrical conductivity, or decrease the resistivity, of the substrate. However, conventional ZnSe bulk single crystals produced by the physical vapor transport (PVT) method and the grain growth (GG) method have high resistivities because they are undoped (no donor impurities are doped).
The resistivity of a ZnSe single crystal can be reduced by heat-treating it in a Zn—Al melt. An example of this method has been proposed by G. Jones and J. Woods in a paper included in J. Phys. Vol. 9, 1976 on pp. 799-810. According to this method, Al diffused in the ZnSe crystal acts as a doner impurity, and the concentration of Zn vacancies decreases concurrently. These phenomena can increase the activation rate of the Al and the concentration of the n-type carriers. As a result, the targeted resistivity can be achieved.
However, this heat-treating process cannot prevent the melt from adhering to the ZnSe single crystal at the time of cooling. Consequently, the stress caused by the difference in thermal expansion between the ZnSe and the melt increases the dislocation density of the ZnSe single crystal, notably decreasing the crystallinity. As a result, the light-emitting device produced by this process poses a problem of life-time reduction.
To solve this problem, Japanese patent 2839027 has disclosed a process in which a ZnSe single crystal having a thin Al film formed on its surface is placed in a hermetically sealed quartz container to be heat-treated in a zinc vapor atmosphere. This process can suppress the notable increase in dislocation density for a substrate having a dislocation density of at least 5×104 cm−2 before the heat-treatment.
However, this process has a problem in that the dislocation density increases with increasing thickness of the Al film when a substrate having higher crystallinity with smaller dislocation density is heat-treated under the same conditions. More specifically, when a substrate having a dislocation density of less than 5×104 cm−2 is used for an Al film having a thickness of at least 40 nm, the dislocation density increases. This increase can be prevented when the thickness of the Al film is decreased. However, the carrier density of the ZnSe substrate after the heat-treatment depends on the thickness of the Al film before the heat-treatment. Therefore, when the thickness of the Al film is decreased, the carrier density decreases and the resistivity of the ZnSe substrate increases. More specifically, according to the conventional process, when the thickness of the Al film is decreased to less than 40 nm, the carrier density decreases to at most 6×1017 cm−3.
In other words, the conventional technology has a problem in that when the thickness of the Al film is increased to reduce the resistivity by increasing the carrier density, the dislocation density increases and when the thickness of the Al film is decreased to reduce the dislocation density, the carrier density decreases, increasing the resistivity.